This invention relates to antenna feed networks and, in particular, feed networks for satellite antennas.
Typically, a satellite antenna comprises a cluster or array of individual horns positioned to direct individual radio frequency beams onto a reflector which redirects a combined beam to the desired coverage area of the earth.
The feed network for the satellite antenna comprises a transmit network, a receive network similar in construction and operation to the transmit network, and a duplexer (also known as a diplexer) array which is simply a means for allowing the transmit and receive networks to share the same array of horns.
Within the transmit (or receive) network is a plurality of couplers which distribute power among the horns in a prescribed manner. Also included in each transmit (or receive) network is a plurality of phase shifters. By varying the line lengths appropriately and by selecting appropriate phase shifters the desired phase relationship among the horns may be achieved.
The phase shifters used are of two types, capacitive and inductive. These give respectively negative and positive phase offsets. The phase offset however varies with frequency. Thus, if a 90.degree. phase difference is required between two lines, a single 90.degree. phase shifter placed in one of the lines will give the correct phase relationship at one frequency only, say at midband; there will be an error at the bandedges. To avoid this error, it is necessary to use a +45.degree. phase shifter in one line and a -45.degree. phase shifter in the other. The two phase shifters, although having differing signs, both have the same phase slope. That is, a capacitive phase shifter having numerically the same phase offset at midband as that of an inductive phase shifter, will also have the same algebraic slope. Thus, a constant phase differential is maintained over a finite bandwidth. In a typical feed therefore, combinations of different capacitive and inductive phase shifters are used throughout.
A typical antenna at K-band may have more than a hundred phase shifters. Because of this large quantity, any simplification in design and/or reduction in size can translate to appreciable savings in cost, volume and weight. To that end, a new component called a phase slope equalizer was developed recently and is the subject of U.S. Pat. No. 4,633,258 issued on Dec. 30, 1986 in the name of Spar Aerospace Limited.
The new phase slope equalizer described in the above identified patent has zero phase offset at midband but has a substantially constant phase slope across the bandwidth.
Phase correction therefore becomes relatively simple. The path lengths of the various feed lines are arranged to give the required phase offsets at midband only and then phase slope equalizers (one per line) are introduced to equalize the slopes among the lines. The slopes of all these equalizers have the same sign. This new approach dispenses with the inductive and capacitive phase shifters.
The advantages of the phase slope equalizer over the phase shifter approach are that the phase slope equalizer is smaller, simpler, less expensive and eases the problem of phase correction. Additionally, fewer phase slope equalizers are required; approximately half the number.
Although the concept of a phase slope equalizer has proved to be an extremely valuable one and has, in practice, given rise to the advantages indicated above, the specific embodiments described in the above identified application were designed as discrete components which require tuning. As antenna technology evolves, a trend towards the use of integrated waveguide at K-band (0.834 cm-2.75 cm) and integrated feed, realized with TEM-line (square coax.) at C-band (3.7 cm-5.1 cm) has become evident and component design has to be compatible with that concept. Key features are that the design be amenable to fabrication as part of an integral assembly and that the design require minimum or no tuning.